Sortable Sessions panel (recency / message count / workspace)

[Ron537]

Summary

Add a sort dropdown to the Sessions panel so users can sort by recency (default), message count, or workspace.

Acceptance criteria

• Sort dropdown rendered in the Sessions panel header (next to or in the existing filter menu in src/renderer/src/components/layout/SidePanel.tsx).
• Sort options: Recent, Most messages, Workspace.
• Selection persists across app restarts via useSettingsStore (new field sessionsSortOrder on AppSettings).
• SessionList receives the sort order as a prop and applies it before rendering.

Files to touch

• src/renderer/src/types/index.ts (extend AppSettings)
• src/renderer/src/stores/settingsStore.ts (default value, migration)
• src/renderer/src/components/layout/SidePanel.tsx (UI control)
• src/renderer/src/components/sessions/SessionList.tsx (apply sort)

How to test

• Verify the default is "Recent" and matches existing behavior.
• Switch to "Most messages" and confirm sessions reorder.
• Restart app and confirm the setting persisted.

Effort: 4-6 hours.